When digital devices are manufactured, they are usually tested to ensure that there are no errors in manufacture. These tests may be performed immediately after manufacture and before distribution of a digital device or, in some cases, during manufacture of the digital device. Due to the advanced miniaturization of modern electronics, testing of digital devices by external monitoring is relatively difficult. That is, it is not easy to determine by external monitoring whether various components are properly functioning and/or properly connected. Therefore, many of the more complex modern electronic devices include testing circuitry as part of the device. The testing circuitry is used at the time of manufacture to perform various tests and, for some devices, it may never be used again. Alternatively, other devices may provide for testing subsequent to manufacture and distribution, such as during service or repair of the device.
The manufacture of devices with internal test circuitry does present some disadvantages. One is that placing test circuits in every manufactured device can result in higher costs per device (as opposed to using external test equipment where a single piece of equipment can be used to test a large number of manufactured devices). For example, if a device includes an integrated circuit implemented on a chip, one or more of the pins of that chip may be dedicated for test purposes. These pins may test, for example, whether the chip is properly connected to a printed circuit board (PCB), whether other elements of the device are also properly connected to the board, and/or whether the various interconnects within the board itself are intact. This type of testing is referred to as boundary scanning. However, for many devices, each pin on an integrated circuit presents a relatively significant cost. Using a pin merely for test purposes tends to magnify that cost, because this means that the pin will not be used through most of the life of the device.